Record temperatures and prolonged periods of drought are increasingly affecting agriculture worldwide. In Germany, too, droughts are now jeopardizing high-yielding harvests. A cultivation system with a new type of irrigation technology using water reuse is particularly suitable for arid regions and selected crops. And now results and specific recommendations are available for a practical application of this alternative type of agriculture that uses recycled water.

Is it possible to minimize the extensive water consumption in agricultural crop production by water reuse? And can valuable nutrients from wastewater be recycled in the cultivation process and used for the production of various vegetables and ornamental plants? In the research project HypoWave, funded by the Federal Ministry of Education and Research (BMBF), scientists have found a way to combine these two goals. In a greenhouse, the interdisciplinary team cultivated lettuce plants in soilless planters. In this so-called hydroponic process, plants are supplied with a nutrient solution. Other than in conventional cultivation, no water seeps into the soil and there is much less evaporation. What is special is that for the first time, treated wastewater is used for this hydroponic plant cultivation. It comes from the neighbouring sewage plant and is treated and sanitised in a multi-stage process. At the same time, nutrients necessary for a healthy plant growth are derived from the wastewater. These can then be recycled, just like the irrigation water itself.

Win-win situation for agriculture and wastewater utilities

“What is special about the soilless hydroponic crop cultivation in our pilot plant is that we were able to successfully optimize the water-saving process already in place by using specially treated wastewater,” says project leader Thomas Dockhorn from the Technical University of Braunschweig. Water reuse not only provides a previously unused water source, he says. The plants in the HypoWave concept are also suitable for more extensive wastewater treatment because they absorb nitrogen and phosphorus. This results in optimal plant growth with a good nutrient supply and at the same time the water itself is purified by this nutrient removal. “The pilot project on the site of the Hattorf wastewater treatment plant near Wolfsburg showed us that the HypoWave process can be a win-win situation for agriculture and wastewater utilities alike due to its sustainability and resource efficiency,” says Dockhorn.

Regional food production despite water shortage

The HypoWave process is particularly suitable for growing vegetables such as cucumbers, tomatoes, peppers or lettuce. A possible place of application in Germany are rural areas with regional water scarcity and wastewater treatment plants without significant industrial discharge. Here the process can contribute to year-round regional vegetable production. However, it is also interesting for cultivation in regions chronically affected by water shortages, such as Spain or Portugal. “Agricultural holdings with an interest in innovation are now called for in connection with the implementation of the HypoWave process,” says project coordinator Martina Winker from ISOE – Institute for Social-Ecological Research. In Germany, there are always opportunities for entering into this sustainable plant production for instance when wastewater treatment plants are being restructured anyway which is for example the case if they have to introduce the fourth treatment step. “This opens up windows of opportunity for the cooperation between farmers and wastewater utilities.”

Risk management for successful cultivation

The most important prerequisite for successful cultivation and a high-yield harvest is comprehensive risk management. “Farmers face the question of whether a risk is connected with the cultivation,” observes Thomas Dockhorn. “Therefore, when choosing a wastewater treatment plant our recommendation is to make sure it is processing wastewater that does not contain any major industrial discharge in order to avoid heavy metal concentrations.” In addition, the microbiological quality of the irrigation water and the products was examined in the project. It turned out that no increased contamination due to the origin of the water could be detected. All in all however, the project recommends a multi-stage risk management that includes hygiene and safety at work as well as additional elements such as UV irradiation of the irrigation water and a strict separation of water and plant spheres.

Landscape concept for a sustainable implementation of the HypoWave process

Two further sustainability aspects that should be taken into account when using greenhouses were explored in the project: “We investigated the fact that production in greenhouses can disturb the landscape and that the ‘isolation’ from nature through cultivation under glass or foil can have consequences for ecosystems,” says Martina Winker. “It is therefore important that such systems are integrated into landscape design concepts and include appropriate compensation measures.” Therefore, the project also developed recommendations on how the HypoWave concept can be embedded into the landscape both aesthetically and with regard to the preservation of sustainable ecosystem services.

Detailed information on the research project is available at www.hypowave.de.

The HypoWave research project
The Federal Ministry of Education and Research (BMBF) supports the joint project “Use of hydroponic systems for resource-efficient agricultural water reuse (HypoWave)” as part of the WavE funding measure. Under the lead of Carolo-Wilhelmina Technical University of Braunschweig (Institute for Urban Water Management (ISWW)), the project partners are ISOE – Institute for Social-Ecological Research, the Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB, the University of Hohenheim (UHOH), the Julius Kühn Institute (JKI), Abwasserverband Braunschweig (AVB), WEB – Wolfsburger Entwässerungsbetriebe, ACS-Umwelttechnik GMBH & Co.KG, aquadrat ingenieure (a2i), aquatectura – studios for regenerative landscapes, aquatune GmbH, BIOTEC Biologische Naturverpackungen GmbH und Co KG and Xylem Services GmbH (Xylem). The project lasted roughly three years and ended on 31 December 2019.